Carter's 15-month sentence for involuntary manslaughter had been on hold pending the ruling.

Defendant Michelle Carter listens to testimony at Taunton District Court in Taunton, Massachusetts, June 8, 2017. Carter is charged with involuntary manslaughter for encouraging Conrad Roy III to kill himself in July 2014.Charles Krupa / AP file

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Feb. 6, 2019, 3:20 PM GMT / Updated Feb. 7, 2019, 5:21 PM GMT

By David K. Li

The Massachusetts Supreme Court on Wednesday upheld the involuntary manslaughter conviction of a woman who encouraged her boyfriend's suicide through text messages.

Michelle Carter will have to serve 15 months behind bars for the 2017 conviction that held her responsible for the July 2014 suicide of Conrad Roy III, 18.

She could begin her sentence as early as Monday, prosecutors said Thursday.

"Her conviction of involuntary manslaughter as a youthful offender is not legally or constitutionally infirm. The judgment is therefore affirmed."

Conrad Roy III, the teen who took his own life with the help of his girlfriend.Courtesy Kim Cardosa

Carter was on the phone with Roy as he inhaled carbon monoxide inside his Ford F-250 truck in Fairhaven, Massachusetts. At the time, Carter was 17 and about a month out of a psychiatric hospital.

She opted against a jury and Bristol County Juvenile Court Judge Lawrence Moniz found that Carter caused Roy's death by telling him to "get back in" his truck as it was filling with poisonous fumes.

The high court cited Carter's own words, in text messages to friends after Roy's death, as supporting her conviction.

For more on this case, watch Dateline NBC Friday at 10 p.m. ET/9 p.m. CT.

"As the defendant herself explained, and we repeat due to its importance, `[The victim's] death is my fault like honestly I could have stopped him I was on the phone with him and he got out of the [truck] because it was working and he got scared and I f--king told him to get back in,'" the court wrote in its decision.

Her 15-month sentence had been on hold until the high court's ruling.

"We are disappointed in the Court’s decision," her lawyers said in a statement. "We continue to believe that Michelle Carter did not cause Conrad Roy’s tragic death and is not criminally responsible for his suicide."

"We will evaluate all legal options including a possible appeal to the U.S. Supreme Court," the statement said.

Her defense team has 28 days to ask the high court to reconsider its ruling. The Bristol District Attorney's Office also needs to go back to a lower court and have the stay on Carter's sentence revoked.

She's been ordered to appear in a Taunton, Massachusetts, courtroom on Monday when prosecutors will ask Judge Moniz to revoke the current stay against Carter's imprisonment, officials said Thursday.

The district attorney wants Carter to be locked up immediately — though the judge could set another hearing or another day for her to surrender, according to prosecutors.

But despite the numerous procedural hurdles still ahead, Bristol County District Attorney Thomas M. Quinn III said his office was "very pleased" Wednesday with the high court's ruling.

"This case is a tragedy for all of the people impacted by this case," Quinn said in a statement. "As the court stated, the defendant herself admitted that she caused the death of Conrad Roy by her own conduct."

Carter’s defense had also drawn support from the American Civil Liberties Union of Massachusetts, which argued that her actions were First Amendment-protected free speech.

And Carter's lawyers had argued to the high court that no one can be convicted of manslaughter for simply instructing someone to harm themselves.

But the state Supreme Court on Wednesday cited three cases that defendants were held criminally responsible for encouraging a suicide.

"In sum, our common law provides sufficient notice that a person might be charged with involuntary manslaughter for reckless or wanton conduct, causing a victim to commit suicide. The law is not unconstitutionally vague as applied to the defendant's conduct," the court said.

Editor's note: If you are looking for help, please call the National Suicide Prevention hotline at 1-800-273-8255.